Method of embedding transformers



Nov. 29, 1955 L. M. MERRILL ETAL 2,724,369

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METHOD OF EMBEDDING TRANSFORMERS Original Filed Dec. 10, 1949 5 Sheets-Sheet 3 Nov. 29, 1955 L. M. MERRLL. ETAL METHOD OF EMBEDDING TRANSFORMERS Original Filed DeG. l0. 1949 5 Sheets-Sheet 4 mmh Mmm

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METHOD OF EMBEDDING TRANSFORMERS Original Filed Dec. l0.- 1949 5 Sheets-Sheet 5 United States Patent O ice 2,724,869 METHOD or EMBEDDING TRANSFORMERS of New Jersey Driginal application December 10, r1949, Serial 132,335.. 'Divided and ythis application May 2 1, 1954,

SerialNo. 431,3 33

This applicationis a division lof `zoperirgiing application Serial No. 132,335 filed December l0,l 1949, now abandoned.

The present invention relates to transformers and `has particular reference to .transformers intended for use in Circuits for lighting and other purposes of the kind v in which a number of y transformers are arranged with their primary windings Aconnected ,at spaced lintervals in a common supply or power circuit and with their .secondary windings connected respectively to different energy con- Asum'ifng units such as incandescent lamps. Such circuits, in which the transformers are often referred to as isolatioii transformers, Vhave long .been employed for street lighting and within more recent years have been extensively employed in airport lighting for runway and abprgsh ylisht Systems and the like In the majority of such installations the cables of the power circuits and the transformers have been buried vin the l ground `with the transformers removably housed in vaults of metal, ceramics, concrete and Vthelike `and with the cables .either eo ned in permanent ducts of similar materials c onnecting the several vaults, ordirectly yburiedin theground between vaults. Such systems .are very expensive, partioularly as to installation costs and are moreover relatively expensive in maintenance due to ultimate leakage of ground Water and the like into the vaults, with re; 4sult'ant electrical breakdowns traceable ,to .corrosion of 'metallic transformer cases or seepage of water into the cable entrance of the transformers.`

Also, incident to modern military operations, advanced and temporary airfields are necessary for which runway lighting systems are required. For such u ses the kinds of installations described above obviously cannot be used, and While SG Called Portable lighting.St.S for1smp0ra-ry installation above ground have been Vdeveloped/ they have not proved to be wholly satisfactory in all particulars and are entirely unsuited for permanent installations.

The general object of the .present invention is `to yirnprove upon apparatus of the kind under` discussion through the provision of a novel form of transformer unit, together with novel apparatus and methods for the manufacture thereof, which will enable isolation vtranslformers for lighting and other purposes to lbe yprovided which are capable of vault installation or direct ground burial Vfor permanent installations, or which may be laid above ground .where they are exposed to atmospheric conditions, without physical or electrical deterioration vover indeterminate periods of time; `which are :lighter than-present apparatus; and which are capable of A'withstanding the abusive treatment usually unavoidable in military operations.

From the foregoing discussion itwill be evident that the invention is directed to transformer units having capacities for transforming substantial amounts of elec- -trical energy in terms of watts or kilowatts and such transformers will hereinafter be designated as power transformers, without limitation as `to specific type or 2,724,869 Patented Nov. 2 9, 1955 design, in order to distinguish `them from .transformers ofthe kind ,utilized in electronic and other applications rof the kind which require transformer capacities of only a fraction ,of a .watt or at most only a very few watts.

To the .end of accomplishing the above and other and .more .detailed .objects hereinafter appear-ing, the ,invention contemplates the provision of a v transforrmer unit r-the core and windings of which are directly encased in a mass of nonfmetallic material of .suitable nature, hereinafter more fully discussed, which is molded around and into the voids of the transformer structure [to .provide a casing Wholly enclosingthe .transformer structure and with the casing bonded to ne sheathing Q f the .input vand output cables providing vthe conductors to and from the transformer windings 'to provided a :hermetic sealibettween the casing and sheathings, the .electrical connec- .tions between the csudufors and the Wires ef the wisdings being located within and enclosed by the molded casing material, Within the framework of Ithis general structure the invention contemplates numerous additional novel and ladvantageous structural features, the nature and advantages 2of which together with the methods and means employed to yprovide rthem will best be understood from the ensuing portion of this specification,

ltaken in ,conjunction with the accompanying drawings in ywhich .different structural embodiments of the invention and kthe manner ofvfabrication thereof are disclosed .by .way of example but without limi-tation, the scope of .the invention being ydefined by the appended claims.

Y11,1 the drawings:

Fig l is a -top `plan view, partly 4in section, of a -transformer `unit embodying the invention;

Fig. 2 is a vertical longitudinal view, partly ,in elevation and partly in section, ofthe u nit shown in Fig. =1F;

Fig. 3 `is a vertical transverseview, `partly in elevttiitcn and partly in section, of the unit rshown kin Fig; l;

fFig. 4 is van exploded view partly :in section .and partly in velevation, ofmolding apparatus for producing atransformer u nit embodying the invention, with a transformer fn place preparatory to the molding operation;

Fig. 5 Ais a top plan lview of the lowermold ,block of rtg. 4; yf

Fig. 6 isahorizontal ,longitudinal sectional view,..`p`artl`y in elevation, of a different example of transformer unit v.en xbodying the invention;

Fig. 7 is a vertical longitudinal sectional view, partly :in eleyation, of the kunit showin Fig. 46'; v Fig. 8 is a vertical `transverse section, vpartly .in elevtion, oftheunit Vshown in Fig..6;

Fig. l9 is a vertical section Athrough molding Capparatus, with .transformerin place, for molding the formof trans'- ,formershown in Figs. 6.-8;

Fig. .10 is a top plan viewof kthe lower block of the molding apparatus .Shown .in Fis- 9; f

vl1 is an exploded elevation on smallerscale ofthe molding apparatus of Fig. l0;

Fig. 1 2 is -a vertical transverse View, partly in elevation and partlyjin section, of afurther example oft'rans'former unit I embodying Ithe invention;

Fig. 13 is afverticallongitudinal view, partly in eleva'- tion and partly in sec-tion, yof the unit showniii Fig. .12; ig. .14 lis afragrnentary section of llarg'erseale,tialtei't on line 14 14 of Fig. 15; and

Fig. :15 is a plan view, partly in section, ofthe unfit v,show nin Fig. 1 2.

Referring now to Figs. 1 to 3 ofthe drawingsa transformer unit embodying the invention i s indicated ,generall'y at 10. In the lembodiment illustrated.thetransformer core 1 2 is formedin known manner by anumberof plate-like metal laminations Y14 held in assembledlrelat'on byrivets 16 andhaving suitable apertursthroughyvhich the `coils of the primary and secondary windings of the transformer pass. Since the specific electrical properties of the transformer and the details of the windings are not germane to the present invention the windings are shown generally at 1S, it being understood that the wires Of these windings are suitably insulated by lacquer coatings or any other form of insulation usual for the purpose. In the persent example, the terminal ends of the primary winding are indicated at and 22, respectively, while the terminal ends of the secondary winding appear at 24 and 26. Cables 28 and 30 provide for the supply of current to the primary winding through metal conductors 32 and 34 connected at 36 and 38 respectively by soldering or any other suitable electrical connecting means to the ends of the primary winding. Similarly the conductors 40 and 42 are connected at 44 and 46 to the ends of the secondary winding. In the present instance the primary conductors are carried in and form parts of separate cables, while the secondary conductors are carried in and form parts of a common cable 48.

In accodance with one of the aspects of the invention the several cables are mechanically anchored to the structure of the transformer so that the windings of the transformer and the joints between their terminals and the cable leads are relieved of all mechanical stresses such 'as tension created by pull on the cables and forces tending to displace the parts during the operation required to form the casing, as is also the juncture between the cables and the casing enclosing the transformer.

In the present embodiment this is accomplished by the provision of anchoring means in the form of what may be termed a harness, by means of which the ends of the cable are firmly located mechanically with respect to the core of the transformer. This means comprises a pair of eye members 50 and 52, preferably of non-metallic material having relatively strong physical properties and high dielectric strength, such for example as Bakelite or equivalent material. The members 50 and 52 are suitably apertured to fit snugly over the projecting portions of the core at opposite sides of the transformer coil and at one end having projecting tongues 54 and 56 which in turn are provided with apertures 5S ano 60 through which extend the tongues 62 and 64 of a transversely extending anchor bar 66, advantageously of the same material as the eye members. The parts are preferably held in assembled relation by cotter pins or the like as seen at 68. The bar 66 is suitably apertured to permit the cables 28, and 48 to pass through it in spaced relation for connection to the ends of the transformer windings. At the places where the cables pass through the anchor bar they are firmly fixed relative to the bar. In the present instance this is accomplished by means of metal clamping rings such as rings 7) and 72, which are crimped around the cable 48 on opposite sides of the bar to firmly clamp it against movement relative to the bar and to transmit to the latter tension or compressive forces between the cables and the transformer. As will be evident, with this construction the joints between the leads and the windings are relieved from all mechanical strain as are also the terminal end portions of the transformer windings, the latter being particularly important because of the fact that in some instances the wires of the windings are relatively fine with little mechanical strength. Similarly, the primary cables 28 and 30 are anchored by means of clamp rings 74 and 76, and 78 and 80, respectively.

After the above described parts are assembled, the unit is then completed by forming around the transformer structure and the ends of the cables a casing 82 in which the transformer is hermetically sealed and to which the cables are also bonded to provide a hermetic seal between the cables and casing.

In order to achieve the best results and to take full advantage of the potential benefits of the invention, consideration must be given to the material of which the casing is formed.

Primarily, the material must be moldable and in order to be fully satisfactory should possess certain additional properties of which the following are important. It should be non-porous to both gases and liquids and should be substantially impervious to acids and alkalis of the kind and concentrations ordinarily encountered in earth installations. It should be substantially impervious to deterioration from the action of the atmosphere, particularly with reference to sun rays. It should be capable of resisting, without adversely affecting its mechanical and electrical properties, extremes of temperature conditions from subzero temperatures to the high temperatures encountered in above ground installations in tropic or desert locations. It should be relatively hrm and nonfranigble and preferably have a certain degree of elasticity. It should be as free as possible from inherent aging with time. It should have reasonably good physical properties, particularly the quality of toughness and should have relative high dielectric strength. Furthermore it must be compatible with the covering of the cables to which it is bonded, to the extent required to provide a hermetic bond between the casing and the cables. Because of the latter factor, the materials of which the cables are made must be taken into account when selecting the casing material. As previously mentioned, flexible cables suitable for direct ground burial have previously been employed and in the development of such cables it has been found that in the present state of the materials art, the class of materials which most satisfactorily meets requirements of the nature of those noted above are synthetic polymers of compounds productive of products generically known as synthetic rubber, of which the following may be mentioned as examples: GR-S (Government Rubber, Styrene) a polymeric product of butadiene and styrene; Low Temperature Rubber, also a polymeric product of butadiene and styrene; neoprene, which is a generic term for a number of chloroprene polymers of different grades and designated as GR-M plus numerical designations for the several grades; Butyl rubber a copolymer of isobutylene and isoprene; and Buna-N (GR-A, Government Rubber, Acrylonitrile) a copolymer of butadiene and acrylo nitrile. To those skilled in the materials art it will be evident that a substantial variety of materials suitable for the purpose are available and that they need not be limited to synthetic rubbers in order to carry out the invention. Such other materials may include synthetic resins such as Polyt'nene Also, while it is possible to mold a casing from natural rubber which can be bonded to cables covered with compatible material, the use of natural rubber is not to be recommended because of its inherent deterioration with time, or so-called aging characteristic. The specific material employed will therefore be dictated by specific conditions in different cases, one of which is the covering material of the cables to which the transformer casing must be bonded.

Of the foregoing materials, neoprene is one of the more satisfactory and for purposes of further description herein it will be assumed, without limitation, that neoprene is the material employed.

Also it is to be noted that in the production of cables of the kind under consideration the covering for the wires is often different from that of the outer covering, as for example a core of natural rubber or other insulating material covered by a sheath of neoprene or other material having the desired physical properties noted above. Since the specific internal construction of the cables is not ger- `rane to this invention, the term sheathing as employed herein is to be considered as referring to the outer covering of the cables regardless of whether that constitutes the sole insulation for the conductors or other insulating materials are used internally ofthe cable.

In accordance with one of the aspects of the invention, the casing 82 is formed by molding the casing material under pressure around the transformer structure and the ends of the attached cables, with the assembly enclosed in a suitable mold cavity, so that not only is the enclosing casing formed and bonded 'to the cables but vids in 'the structure are filled with the molded material, the cavity being suitably vented to permit the escape of air or other gases from the cavity. The pressure molding is ordinarily carried out at elevated temperature but the temperature factor is dependent upon -tihe specific nature ofthe maferial being molded, as hereinafter more fully discussed.

While the moldingy operation may be carried out with different forms of apparatus we have found pot molding apparatus, of the kind n'ow to be described, to be satisfactory for the required purpose.

Referring now to Figs. -4 and 5, Aa suitable form of such apparatus is shown .i-'n which the mold comprises upper and lower mold or die blocks S4 and r86, formed with confronting and mating ycavities 88 and 90, respectively, which when .the blocks are together form a cavity for the transformer structure. The transfer-mer structure is held in xed spaced relation to ,the walls of the cavity by suitable centering means, which yin the example shown comprise four studs 92 projecting upwardly in the cavity and shouldered to provide small centering pins 94 `adapted to .project into suitable holes in the core 12 which vrests on the shoulders. Similar studs 96 provid-ing centering pins 98 ,project `downwardly from the upper block 86. The .upper .and lower rblocks are each provided with semicircular grooves leading from the respective recesses 88 and 90 to the exterior of .the blocks, these grooves mating to provide .passages for vthe reception of the cables leading to :the .transformer when the blocks are closed with a transformer in place, as seen at 100, 102, and 104 in Fig. 5 and at106 and 1 08 invFig. 4.

,The -wfalls yof the grooves 100, 102, .and v104 are trans- -yersely ridged by seri-ations or corrugations, as seen more clearly in Fig. l5, it being understood that the walls of the Agrooves such las 106 and 108 in the .block 86 are similarly formed. Advantageously the transverse ridges cmay `be formed by a helical thread having a rounded or `other blunt crest. The diameters of the ridged grooves aresuch that when the blocks are closed for the molding operation, the ycables are tightly clamped by the .ridged sur-faces lto prevent axial displacement of the cables relative to the blocks. Guide pins 112 projecting upwardlyfrom the lower block 'and adapted to enter holes 114 `in the upper block may also be provided for holding the cables in position until the blocks are closed.

rIhe lower block is further provided with a vent passage 116 lleading from the cavity 88 to the exterior of the block.

The upper block `is provided with depending 'studs 1118 `adapted to enter mating holes 120 in the lower block to center the blocks when they are closed and may further be provided with handles 122 ,for manipulating the block. The upper face of the upper block 86 is recessed to provide .a cavity 124 for the Vreception of the material -to be molded, and this cavity is connected with the mold cavity 'by lsprue passages 126 and 128 which at their lower ends communicate with the recess 90 by way of the sprue grooves 130 and 132 cut in the lower face of block 86. A'pressure member 134, advantageously having manipulating handles -136 is formed with a depending plunger orpiston portion 138 shaped to mate with the cavity 124 (both advantageously being circular) and is further provided with depending centering studs 140 adapted to enter mating holes 142 in block 86.

v The general manner in which the molding operation is effected 4willlargely be evident from the drawings. The transformed with its attached cables is rst placed in properiposition in the cavity of the lower block and the upper and lower blocks brought together. A suitable quantity `of the material to be molded is placed lin the cavity 124 and the pressure member 134 placed over the upper block 86. The assembly is then placed in a suitable press, indicated in the drawing by lower and upper platens 144 and 146. The required pressure is then applied and the molding material is forced by the plunger 13s 'out of cavity 124 through the-spine passages into the mold cavity, forcing air and/or gases' :out of the cavity and the voids in the transformer 'structure through the vent 116, and filling the cavity to form the desired casing and 'also lto Afill Vthe said voids. Since in orde'r to insure the desired yfilling of the voids ythroughout the transformer structure, high pressures of the order of a ton or more per square inch lare employed, the. ridge'd clamping passages 4for the cables are employed to prevent internal pressure developed in the mold `cavity from forcing the cables out of their respect-ive channels lin the die.

While the `above describes in gener-al the pressure molding operation, other steps must be taken in connection therewith in order to achieve satisfactory results when using a preferred material such vas neoprene which requires heat as well as lpressure for proper molding. To supply the required heat, `the die block-s and pressure member are maintained 4'at elevated temperature, advantageously of the order Tof 300 1F. .for neoprene molding. Such heating may :be effected by v'any suitable means, as for example by conduction from hot press platens or by direct heating by electric resistan'ces or the like.

Also, it is important that prior to the molding operation the transformer structure be heated to desired .elevated temperature and "soake'd for a period suli'ci'ently long for the temperature to become stabilized at the desired value. Such preheating serves several purposes, Aamong the most important of which are .to insure the driving .oif of any moisture in the structure, to enable :the molded material to thoroughly permeate the structure and ll the many voids therein l'without risk of cold shuts, and .tofshorten the time of :the ymolding cycle. The latter may be particularly important with certain molding materials ywhich are susceptible to overcur-in`g with deterioration of vdesirable physical land electrical properties if subjected to molding temperature for .too long `a period `of time as might be required if the transformer unit were not preheated.

The stabilized preheat temperature to which .the transformer is heated will of course 'depend upon the ftem.- perature characteristics of the material Vto be molded. In general the preferred procedure is to `preheat to at least approximately Vdie block temperature and in the event the molded material is of a variety moldable fat relatively low temperature .the transformer still should be preheated to a `temperature capable of thoroughly driving off moisture.

Since the preheated .transformer-is immediately molded, the cables are attac-hed before the preheating operation, and the method of heating must be selected so that the properties of the attached insulated cables are :not impaired electrically .or mechanically. It has been found that placing the assembly of transformer and ,attached cables in an oven at proper temperature and for ysufficient time to properly soak yit .does change the properties .of cables of the kinds usually employed, whereas if the transformed is heated by other methods which substantiallyfconfine theiheating to the transformer itself, damage to the cables is avoided. Accordingly, .the transformer is ,preferably preheated by some such other method, as for .example by placing the transformer between heated blocks in contact with the core, to which heat is transmitted by conduction. Other methods may be employed, such as by high frequencylinduction heatingand also k,by passing current through the windings at substantially higher frequency than that for which the transformer is designed, with consequent lheating due to increased hysteresis losses. While .theconductors of the cables may have some temperature rise due to conduction from the heated parts `to which lthey are attached, such heating is negligible in effect when methods such as those described are employed.

Furthermore, before the transformer structureis ,placed in the die, the end portion of the cables which extend into the cavity are suitably prepared to enable the required hermetic bond between the cables and the molded material to be effected. Such preparation will naturally be different with different materials. In many cases where the cable sheathing and the molded material are the same, direct fusion may be obtained without preparation other than proper cleaning of the sheathing and roughening of the surface. In other instances, pretreatment of the cable ends with a suitable bonding agent which may be of a solvent nature with respect to the materials used, may be required. As previously noted, the molding material must be compatible with the cable sheathing, and as herein employed the term compatible is intended to include not only materials directly fusible to each other but materials which by the aid of a bonding agent or agents may be hermetically bonded together, as distinguished from being mechanically joined.

After the molding operation is effected the molded unit is left in the hot die cavity, for a period of time suiiicient to enable the molded material to set and to cure,

the period of time varying with the specific nature of the material and the size of the transformer. A curing period of thirty minutes for a transformer unit requiring the injection of about three pounds of Neoprene has been found to be satisfactory.

After the curing period, the die blocks are separated, and the unit removed. When removed from the blocks the unit is left with a number of cavities leading from the exterior to the core as the result of the withdrawal of the locating studs and pins required to hold the transformer structure in fixed position during the molding operation. These holes are plugged with plugs of the molding material, a bonded joint being effected by a solvent or other appropriate means. Since the cavities to be plugged are blind, the insertion of the plugs is facilitated by evacuating the air from the cavities. This is acomplished by inserting a very fine hollow tube into the passage along one side as the plug is inserted, the tube being connected to a suitable vacuum pump, and

then withdrawing the tube before the bond between the parts has set, so that the minute aperture left by the tube is closed by the resilient action of the material.

The invention is readily applicable to many different forms of transformer structures and by way of example the application of the invention to a transformer having a coil type core is illustrated in Figs. 6 to S. In this structure the core 150 is formed by a strip of sheet metal wound upon itself to provide an annular core through the central aperture of which the windings of the primary and secondary coils of the transformer, designated generally at 152, pass.

A pair of anchoring members 154 and 156, of baltelite or the like, are rigidly clamped to the core by means of bolts 158 and nuts 160. The primary cables 162 and 164 are anchored to member 154 by means of metal clips 166 and 168 held by screws 170 threaded into member 154 and are also anchored to member 156 by similar clips, one of which appears at 172 in Fig. 7. inwardly of the second set of clamps the conductors 174 and 176 of the cables are joined respectively to the ends 17S and 180 of the primary winding by suitable joints indicated at 182 and 184. The secondary cable 186 is fixed to the member 154 by a metal clamp 188 held by screws 19() and on the inner side of the clamp is bent sharply upwardly to pass through a suitable hole 192 in member 154, which appears in dotted lines in Fig. 8. Cable 186 carries both secondary conductors 194 and 196, which are connected respectively to the ends 198 and 200 of the secondary winding by means of suitable joints 202 and 294. As will be evident from the foregoing, the cables are firmly harnessed to the core, to which external stresses such as result from pull on the cables, are transmitted, thus relieving the conductors, joints and windings of strain.

ln Figs. 9, 10, and 11, suitable molding apparatus is shown which in this instance comprises a lower block 296 recessed at 208, an intermediate block 210 having a through-recess 212 and upper block 214 having a recess 216 and a pressure member 218 having a plunger 220 adapted to enter a second recess 222 in the upper block. Shouldered studs 224 provide locating pins 226 which enter holes 228 (Fig. 6) formed in the anchor members 154 and 156. When closed the blocks are centered by studs 230 and 232 projecting from the upper and lower blocks respectively and entering bores 234 in the intermediate block, and the recesses 208, 212 and 216 together form the die cavity, which is connected with the material receiving cavity 222 in the upper block by the sprue passages 236 formed in the intermediate and upper blocks. Channels 238, 240, and 242 are provided by mating grooves in the lower and intermediate blocks for the reception of the cables, the walls of the passages also preferably being provided with transverse ridges to engage the cables in a compressive grip. The lower block is grooved to provide a vent 250 and there is also preferably provided a shallow recess 252 into which excess material is extruded during the molding operation to form a flap which can be sheared oft after the transformer unit is removed from the mold.

So far as the molding operation and the steps incident thereto both before and after the molding are concerned, the previous discussion in connection with the rst embodiment described applies and therefore need not be repeated in connection with the present example.

As previously noted, the specific nature of the transformer, particularly as to its electrical characteristics, is not germane to the present invention, but for best results certain factors of design and construction should be given consideration. The design should be such that the molded material should have access to all current carrying surfaces and the connections between the conductors of the cables and the transformer windings should be of a permanent nature, as by soldering or brazing, and the joints should be free from sharp projecting edges or points productive of corona effect. Further, the arrangement should be such as to place the greatest practical amount of insulating material between different conductors at different potentials.

To this end the anchoring means may be constructed to provide an additional insulating or dielectric barrier between the joints at the ends of the several conductors and in Figs. 12 to 15 another form of unit is shown which employs a transformer of the same construction as shown in Fig. 1 but in which the means for joining the conductors to the windings and for anchoring the cables is somewhat different from the previously described embodiments.

In the present example the transformer has a at laminated core 260 and associated primary and secondary windings indicated generally at 262. At two corners, the laminations may be secured by rivets 264 and at the remaining corners they are held by through bolts 266 and nuts 268. Two sheet metal loops 270 and 272 have apertured ends through which the bolts pass to fix the loops to the armature. The midportions of the loops pass in front of the end portions of an anchor member 274 made of relatively strong material of high dielectric strength. Advantageously this may be of a molded synthetic resin, nylon being a suitable material. In the present instance the member 274 is formed with a main at web 276 having apertures 27S and 280 at its ends through which hollow rivets 232 and 284 pass to rivet the member to the loops 270 and 272. Intermediate its ends the member is formed with a series of spaced laterally projecting anges or barriers 236, there being five in the present instance, and between each two barriers the web is apertured and a hollow metal gromrnet or hollow rivet 288 passing through each aperture is riveted to the web, as seen more clearly in Fig. 14. Under the outer flange of each rivet 288 a metal connector 290 is secured, the connector having at one end an apertured tab portion through which the rivet passes and at its other end having a socket for the reception of the coil wire to which the connector is soldered or brazed to provide a permanent joint. The member 274 is further provided with laterally extending strengthening webs 292 and 294 and the forward face of web 276 is advantageously bev elled at 296 between the several barriers to avoid the necessity for sharp bends in the tab portions of the connectors. As in the previous embodiment, the primary cables 298 and 300 are spaced apart with the secondary cable 302, carrying two conductors 304 and 306, between them. As shown in Figs. 14 and 15 the two conductors are separated to pass through adjacent rivets 288 to which they are soldered or brazed to form permanent connec tions, the conductors preferably being headed over at the inner ends of the rivets to provide greater resistance against being pulled out. The ends 308 and 310 of the secondary winding are soldered or brazed to the appropriate connectors 290. Similarly, the ends 312 and 314 of the primary winding are fixed to the connectors associated with the rivets to which the primary cable leads are fixed.

As will be evident from the drawings, the several cables are firmly anchored to the transformer core through the medium of the metal conductors, and a positive and permanent electrical connection is established between the conductors and the transformer windings. Also as will be evident from the drawings, the flanges or barriers 286 between adjacent joints provide means in addition to the molded material for isolating the joints from each other to minimize the possibility of electrical leakage within the transformer between joints at substantially different potentials. Since the configuration of the transformer or unit shown in Figs. 12-15 is essentially the same as that shown in Figs. 1-3, it will be obvious that insofar as molding apparatus and procedure is concerned, that shown and described in connection with Figs. 4 and 5 is applicable.

While in the foregoing, pot molding operations have been described, by way of example, in which all of the molded material is forced from one mold cavity to another in which the transformer is located, it is to be pointed out that since in most instances a relatively thick casing is formed around the transformer, and the molding operation is ordinarily carried out at high temperature and accompanied by a subsequent curing period at high temperature, part of the required quantity of molding mate rial may be packed around the transformer in the mold cavity, in sheet or other form, before the mold blocks are closed, so the entire quantity of molding material required for the casing does not have to be transferred by the molding operation. Also, it will be understood that insofar as the invention is concerned, injection molding machines of the kind used for plastics molding may be employed.

From the foregoing it will be evident that many different specific forms of transformer units may be produced through the use of numerous different specific forms of apparatus and modes of procedure applied to a variety of materials, without departing from the principles of the invention the scope of which is to be understood as embracing all articles, appartus and methods falling within the purview of the appended claim.

What is claimed:

The method of making a transformer having a corecoil assembly and a protective casing of molded nonmetallic material around said assembly and cables extending from the transformer assembly comprising metallic conductors connected to the coils of said assembly and covered by sheathing of non-metallic protective material, by the aid of a mold comprising separable die blocks providing therebetween a mold cavity and passages extending from said cavity to the exterior of the mold, which includes the steps of xing the core-coil assembly, together with the metallic conductors of said cables connested to the terminal ends of the windings of the coils of said assembly, with the core-coil assembly in said cavity in predetermined relation to the walls thereof and with the connected cables extending through said passages with an appreciable length of the sheathing of said cables located within and having ends terminating within said cavity, forcing a moldable casing material compatible with the material of said sheathing into said cavity under pressure to form the desired casing extending around said assembly and around the portions of said sheathing exposed within the cavity to bond the material forming the casing to the sheathing of the cables, and clamping the sheathing of the portions of said cables passing through said passages in the walls of the mold to prevent extrusion of thesheathing from the die cavity outwardly through said passages due to pressure exerted by the molding material forced into the cavity on the exposed ends of the sheathing within the cavity.

References Cited in the le of this patent UNITED STATES PATENTS 1,333,004 Vaughn Mar. 9, 1920 1,982,279 Apple Nov. 27, 1934 2,391,038 Rifenbergh Dec. 18, 1945 2,413,897 Wilson Jan. 7, 1947 2,534,119 Gethmann Dec. l2, 1950 

